The explosive growth in mobile electronic devices such as smartphones and tablets creates an increasing need in the art for compact and efficient switching power converters so that users may recharge these devices. A flyback switching power converter is typically provided with a mobile device as its transformer provides safe isolation from AC household current. This isolation introduces a problem in that the power switching occurs at the primary side of the transformer but the load is on the secondary side. The power switching modulation for a flyback converter requires knowledge of the output voltage on the secondary side of the transformer. Such feedback can be obtained through opto-isolators bridging from the secondary side to the primary side but this adds to cost and control complexity. Thus, primary-only feedback techniques have been developed that use the reflected voltage on the primary side of the transformer in each switching cycle.
In a switching cycle for a flyback converter, the secondary current (the current in the secondary winding of the transformer) pulses high after the primary-side power switch is cycled off The secondary current then ramps down to zero as power is delivered to the load. The delay between the power switch off time and the secondary current ramping to zero is denoted as the transformer reset time (Trst). The reflected voltage on the primary winding at the transformer reset time is proportional to the output voltage because there is no diode drop voltage on the secondary side as the secondary current has ceased flowing. The reflected voltage at the transformer reset time is thus directly proportional to the output voltage based upon the turn ratio in the transformer and other factors. Primary-only feedback techniques use this reflected voltage to efficiently modulate the power switching and thus modulate the output voltage.
One issue, however, with primary-only feedback occurs during low-load or no-load periods of operation. The primary-only feedback controller in the flyback converter detects this lack of activity on the secondary side of the transformer and stops cycling the power switch accordingly so that the secondary side is not driven out of regulation. Such lack of pulsing is satisfactory so long as the load remains dormant. But should the load again be applied, the controller has no way of detecting this reapplication of the load without a current pulse being generated to produce reflected voltage on the primary side (for example, as sensed through a primary-side auxiliary winding).
To solve this problem, an activity detector has been developed for coupling to the output diode on the load side of the converter. The activity detector is configured to generate a secondary winding current pulse despite the power switch continuing to be dormant. An example of such an activity detector is provided by commonly-assigned U.S. application Ser. No. 14/340,482, (the '482 application) filed Jul. 24, 2014, the contents of which are hereby incorporated by reference in their entirety. This secondary-side activity detector detects the termination of a secondary current pulse as generated conventionally from a cycle of the primary-side power switch. After this transformer reset time, the voltage across the auxiliary winding will oscillate due to the resonant circuit formed by the inductance of the transformer and the parasitic capacitance of the power switch. Since this oscillation could be interpreted by the controller as the application of a load (or occurrence of a fault condition), the secondary-side activity detector delays after the Trst time for a “blanking period” to allow the oscillations to sufficiently subside. Upon the termination of the blanking period, the secondary-side activity detector monitors the voltage difference across the rectifying diode on the secondary side to determine whether a load has been applied. With an applied load, the voltage across the rectifying diode changes as the load capacitor discharges. The secondary-side activity detector detects this voltage difference across the rectifying diode and switches on a low-impedance current path that bypasses the rectifying diode. For example, the secondary-side activity detector may comprise a two-terminal device that couples to the cathode and anode of the rectifying diode. Should the secondary-side activity detector detect a load-induced voltage change across the rectifying diode subsequent to the blanking period, it shorts the cathode and anode of the rectifying diode through its low-impedance alternative current path. This low-impedance current path allows the charged output capacitor on the secondary side of the converter to send a pulse of current through the secondary winding that in turn creates a reflected pulse on the primary-side auxiliary winding. The flyback controller is configured to detect this secondary current pulse. Since this secondary current pulse is not created by the pulsing of the power switch, the corresponding reflected voltage is denoted herein as an “activity signal” to distinguish it from the reflected voltage obtained from a power switch cycle.
In response to detecting the activity signal, the flyback controller cycles the power switch. The resulting reflected voltage may then be used through primary-only feedback techniques to directly monitor the output voltage so that it may be regulated accordingly. Although this generation of an activity signal is quite advantageous, note that the activity detector may need to be configured with operating parameters. It is conventional to program devices with such operating parameters using a write-once memory such as a bank of fuses. A bit of configuration data is written to the device by either “blowing” the corresponding fuse or leaving it intact. To ensure that the configuration of the fuses was done correctly, the configuration data is then retrieved following configuration of the device. Such writing and reading of configuration data conventionally requires at least three terminals: a configuration write terminal, a configuration read terminal, and a ground terminal. But note that the activity detector disclosed in the '482 application has just two terminals. Although this is quite advantageous with regard to reducing manufacture costs, the resulting lack of terminals prevents a conventional configuration of the device.
Accordingly, there is a need in the art for techniques enabling the programming and verification of a memory within a device having just two terminals.